(738f) Investigation of the Surface Reaction Pathways for Catalytic Valorization of Methane over NiO/Ce0.83Zr0.17O2

Methane constitutes the major component of natural gas and is abundant in nature. However, a large fraction of methane resources is not sufficiently utilized due to difficulties in transportation and the economy of scale of methane steam reforming processes. Hence, it is highly desirable to develop efficient methods for methane valorization to products such as higher alkanes, oxygenates and aromatics. Our previous research had shown that NiO/Ce_0.83Zr_0.17O₂ catalysts are active for various methane valorization processes under moderate conditions. To further improve the yield and selectivity towards desired products, such as methanol and benzene, it is crucial to understand the active sites of the catalyst and the surface reaction pathways.

Using in-situ IR spectroscopy, the surface intermediates formed during methane activation were monitored. The formation of methyl/methoxy groups, hydroxyl groups, aromatic species and carbonate species was observed. Additionally, the evolution of different surface species in the course of 12 h was evaluated to elucidate the surface reaction pathway during methane activation. A slowdown of the growth rate of all surface species was observed after 10 mins upon exposure to methane, indicating a change in physiochemical properties of the active sites during the methane activation process. Adsorption of expected products on the catalysts surface was also monitored using IR spectroscopy to identify the key intermediate species during methane valorization. The peaks for aromatic species and C-O vibrations originated from methane activation overlapped with those from benzene and methanol adsorption.

Finally, using in-situ X-ray absorption spectroscopy, the changes in oxidation states and local electronic structures of nickel and cerium were revealed to identify the active sites. Combining the two pieces of information, a comprehensive reaction pathway for methane valorization on NiO/CZ was revealed.